Objective: Group A Streptococci (GAS) are divided in 150 emm-types based on the sequence of the N-terminal 50-amino acids (aa) hypervariable region of the M protein. M is a major virulence factor, anchored in the membrane through a conserved C-terminal region. M is composed of 4 regions (A, B, C and D) formed of repeat blocks. Such structures are substrates for intragenic homologous recombination which may lead to antigenic variation. Epidemiological studies performed in developed countries underlined the small number of different emm-types. However, a larger diversity of emm-types was found in developing countries, notably Brazil. To gain insights into the molecular bases for such difference, the genetic relatedness of the whole surface exposed part of M proteins of Belgian and Brazilian isolates was investigated.

Methods: Multiple alignments of the M sequences (from the N-terminus to a conserved sequence in the D2 repeat; 200 to 435 aa) from 51 representative emm-types (selected among a well-characterised Belgian and Brazilian epidemiological collection by a clustering method) were performed. A phylogenetic tree was constructed using the neighbour-joining algorithm. Several methods were assayed on the same dataset with congruent results.

Results: The prevalence of the Belgian and Brazilian isolates was similar in clades A and B. Clade B contains two monophyletic groups showing geographical preference, sub-clade B1 was mainly composed of Brazilian isolates while sub-clade B2 contained 55% of the Belgian isolates and only 14% of the Brazilian ones. B2 presented the particularity of being composed of isolates that are distantly related. Multiple alignments revealed that (i) the genetic diversity is mostly generated by a high degree of sequence variation in the C repeat region and (ii) by an increase of the number of A and B repeat leading to an overall increase of the size of the M proteins. The Dn/Ds analysis showed that purifying selection drives the evolution of M.

Conclusion: Despite a small number of different emm-types and consequently an apparent low diversity, the overall genetic diversity of the M proteins from the Belgian isolates was comparable to that of the Brazilian ones. The evaluation of the whole surface exposed sequence variations will shed light on the molecular mechanisms involved in virulence and host colonisation as well as the selective pressure driving the antigenic variation and evolution of the M protein.

Genetic diversity of M protein among Belgian and Brazilian GAS isolates. The emm-type of each sequence is indicated next to the bar. Green and red squares indicate Brazilian and Belgian strains respectively. REA indicate a rearranged emm pattern. The evolutionary history was inferred using the Neighbor-Joining method. Bootstrap value higher than 70% are shown next to the branches (500 replicates). The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used for the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method and are in units of the number of amino acid substitutions per site. The rate variation among sites was modeled with a gamma distribution (shape parameter = 0.5). Thirteen clusters (AM) were defined based on an arbitrary defined maximal genetic distance of 0.37 substitutions per site in each cluster. Clade A and B group Sof-positive and -negative emm-types respectively. Sof activity is unknown (*).

Session Details

Date:

19/04/2008

Time:

00:00-00:00

Session name:

18th European Congress of Clinical Microbiology and Infectious Diseases